Control and monitoring of processes involving the flow through tubes of fluids that may be of complex rheology is an important problem in such diverse industrial settings as preparation and processing of food, manufacture of personal care products, oil exploration, extraction, and production, production of coatings, and biotechnology. Ideally, measurements should be made non-invasively and in real time so that defects or variations in quality can quickly be identified and corrected, and all of the parameters of interest would be obtained by a single measurement device. Among the fluid parameters of interest are the flow rate, density, and viscosity. Viscosity in particular is difficult to measure on-line, as particulate matter in the fluid tends to interfere with the operation of most conventional viscometers, and many fluids of interest have viscosities that depend on the shear rate or shear rate history and hence cannot be described by a single-valued viscosity. Construction of rheograms (flow curves) for such fluids requires more sophisticated instrumentation than a simple viscometer, but the instruments generally used for making the measurements needed to construct rheograms, which generally contain narrow gaps or vibrating elements, are unsuitable for fluids that may contain particles or sticky deposits.
A number of systems for measuring fluid density are known that are based on measurement of hydrostatic pressure and means to take into account frictional losses in the pipe. For example, U.S. Pat. No. 2,768,529 discloses a continuous density measurement system based on measuring pressure differences in vertical tubes in series. The differential measurements are arranged such that subtraction of pressure loss due to friction is possible. U.S. Pat. Nos. 3,175,403 and 4,417,474 disclose continuous density measurement systems based on measurement of pressure differences in a vertical tube and a horizontal tube. The horizontal tube permits subtraction of frictional pressure losses in the vertical tube. These instruments only measure density, however, and do not provide information about rheology or flow rate. Furthermore, the methods disclosed in these patents require that all of the tubes be of the same diameter, since the rheology of the fluids is unknown.
Coriolis-type flow meters an example of a type of instrument well-known in the art for the measurement of both flow rates and density. These instruments, while multi-parametric, do not measure rheology, and are susceptible to errors if the mass of the vibrating element changes due to solid deposits. In addition, at very low flow rates, these instruments have large errors, reducing the range of flow rates that can be measured by a single instrument; the instrument must be selected to match the process conditions and a larger or smaller instrument may be needed if the conditions change.
A number of rheometers have been described that are based on non-invasive velocimetry methods. For example, U.S. Pat. No. 5,532,593 discloses a rheometer based on nuclear magnetic resonance. A system based on ultrasound is described in Dogan et al., “Measurement of Polymer Melt Rheology Using Ultrasonics-Based In-line Rheometry,” Meas. Sci. Technol. 16 (2005) 1684-1690. An optically based method is described in Degre et al., “Rheology of Complex Fluids by Particle Image Velocimetry in Microchannels,” Appl. Phys. Lett. 89 (2006) 024104. These systems can measure rheology and flow rate even for difficult systems containing large particles, and a single instrument can be calibrated electronically to cover a wide range of flow rates, but they cannot measure fluid density.
A single integrated system that can perform simultaneous accurate non-invasive in-line, real-time measurements of flow rate, density, and rheology even for fluids of complex rheology or fluids that may contain particulate matter, that does not require that all of the tubes or pipes through which the liquid flows be of the same diameter, and that can perform accurate measurements over wide ranges of flow rates, remains a long-felt, but as yet unmet, need.